Ring strain in boroxine rings: computational and experimental considerations

B3LYP/6-311+G(d) calculations indicate that (HBO)3 (4) and (HBO)4 (5) possess (zero-point energy corrected) strain enthalpies of 11.4 and 31.6 kJ mol−1, respectively. The absence of eight-membered (RBO)4 rings is attributed to a combination of ring strain and the lability of the BO bond. The synthesis, characterization and molecular structure of (PhBO)3·pyridine (1) are described and chemical phenomena related to the addition of amines to triorganoboroxine rings are rationalized in terms of relief of ring strain in 4.